Circuit for protecting a switching transistor

Abstract

A circuit for protecting a switching transistor connected between a direct-current voltage source and an inductive load has a voltage-limiting element connected between the source and the transistor for switching the transistor when the voltage of the source exceeds a predetermined limit, elements for deriving from the voltage-limiting element a control signal when the source voltage exceeds the predetermined limit, and a variable pulse-width modulator connected to a control input of the transistor and receiving the signal from the means. The modulator decreases pulse width when the source voltage exceeds the limit.

We claim: 
     
       1. A circuit for protecting a switching transistor connected between a direct-current voltage source and an inductive load, the circuit comprising: a voltage-limiting element connected between the source and the transistor for switching the transistor when the voltage of the source exceeds a predetermined limit;   means for deriving from the voltage-limiting element a control signal when the source voltage exceeds the predetermined limit; and   a variable pulse-width modulator connected to a control input of the transistor and receiving the signal from the means, the modulator decreasing pulse width when the source voltage exceeds the limit.   
     
     
       2. The circuit defined in claim 1 wherein the control signal is formed by voltages exceeding the avalanche voltage of the voltage-limiting element. 
     
     
       3. The circuit defined in claim 1 wherein the switching transistor is switched when the control signal exceeds a predetermined voltage level. 
     
     
       4. The circuit defined in claim 1 wherein the voltage-limiting element is a zener diode having a cathode connected to an input element of the switching transistor and an anode, the means including a condenser connected to the anode and a discharge resistor connected between the condenser-anode junction and an input of the pulse modulator. 
     
     
       5. The circuit defined in claim 4, further comprising: a current regulator connected at a summing point the condenser and to the input of the modulator; and   a desired-value signal generator connected to the summing point.   
     
     
       6. The circuit defined in claim 4, further comprising a second zener diode connected between the anode of the first-mentioned zener diode and the input of the switching transistor.   
     
     
       7. The circuit defined in claim 6 wherein the avalanche voltage of the second zener diode is such that the condenser can charge sufficiently to set the ratio of the modulator equal to 0.